Summary of Work: Anti-phosphocholine (PC) antibodies are known to provide protection against challenge with virulent Streptococcus pneumoniae. X-linked immune deficient (Xid) mice fail to produce anti-PC antibodies even after immunization with S. pneumoniae. Consequently, Xid mice are extremely susceptible to infection with S. pneumoniae. PC-specific B cells appear to undergo clonal deletion in Xid mice; however, a new thymus-dependent form of PC, 6-(O-phosphocholine) hydroxyhexanoate (EPC), can rescue PC-specific B cells from the bone marrow presumably by providing T-cell help before clonal deletion. Analysis of PC-specific IgG hybridomas from Xid mice revealed utilization of several V-D junctional variants of the VH1 gene segment rearranged to different D and JH gene segments. The majority of Xid anti-PC antibodies exhibit an Asp - Gly 95H replacement at the V:D junction. These Gly 95H VH1 variants associate with kappa1C L-chains to produce anti-PC antibodies that: 1) have low relative affinity for PC; 2) are heteroclitic for nitrophenylphosphocholine; and 3) fail to bind to or provide protection against S. pneumoniae. Single prototypic V-D variants of the T15-id (Asp 95H), M603-id (Asn 95H) and M167-id (Asp 95H - Ala 96H) were also induced in Xid mice. The M603-like and M167-like antibodies bound to and protected against S. pneumoniae even though they exhibited Ka's for PC which were lower than T15-id+ antibodies. These data demonstrate that small changes in the V-D junctional sequence of the T15 (VH1) heavy chain alter L-chain usage and the structure of the PC binding-site so that the PC expressed on S. pneumoniae is no longer recognized. Transgenic mice expressing genes encoding heavy (H) and H + L (light) chains for anti-PC antibodies can be protected against challange with virulent S. Pneumoniae. Following immunization with EPC-KLH, these mice produce only IgM anti-PC antibodies of a single idiotype. In some cases, the level of protection is almost as good as that seen in normal mice which can produce both IgM and IgG anti-PC antibodies following immunization with EPC-KLH. Very little preimmune protection is seen in most of these transgenic mice, although, some H chain transgenic mice have a higher ID50 than normal controls prior to immunization. This suggests that the level of anti-PC in the serum is more important than the number of B cells present that are capable of producing anti-PC antibodies.